Optical amplifier

ABSTRACT

An optical amplifier is disclosed which, in an optical amplifier adapted such that signal light and pump light are propagated through an optical waveguide structure (2, 2&#39;) therein made of an optically nonlinear material to thereby achieve optical parametric amplification or four-wave mixing optical amplification of the signal light, is provided with means for attenuating idler light to be generated within the optical waveguide structure. The optical amplifier has an advantage that phase matching between the pump light and the signal light is easily achieved and thus an optical amplifier operative over a broad frequency band can be realized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to an optical amplifier on theprinciple of optical parametric amplification or four-wave mixingoptical amplification, and more particularly to an optical amplifier inwhich phase matching between signal light and pump light is easilyachieved and, hence, effective optical amplification of the signal lightcan be obtained over a broad frequency band.

2. Description of the Related Art

Optical amplifiers of the type in which the amplitude of electric fieldof light is directly amplified are applicable to the following uses inthe optical fiber transmission system, and of late, intense research onthe optical amplifiers of this type is being made in various areas:

(A) Use for increasing optical output of a light source: By increasingthe output of a light source of the signal light in an opticaltransmitter, the transmission distance can be increased. When theoptical amplifier is used for the light source of local light in anoptical receiver on a coherent optical wave communication system, thereception sensitivity can be improved.

(B) Use for a preamplifier in an optical receiver: By performing opticalamplification in the stage immediately before the photoelectricconversion stage, the reception sensitivity can be improved.

(C) Use for an optical repeater: By the direct amplification of light,as compared with the method in a conventional optical repeater in whicha light signal is once photo-electrically converted into an electricsignal and then the electric signal is amplified, it becomes possible tomake the repeater itself smaller in size and also to increase therepeater-to-repeater distance.

There has been known an optical amplifier in which optical parametricamplification of signal light is achieved by nonlinear effect of secondorder obtained when signal light and pump light are propagated throughan optical waveguide structure made of a nonlinear optical material.

There has also been known an optical amplifier in which four-wave mixingoptical amplification of signal light is achieved by nonlinear effect ofthird order obtained when signal light and pump light are propagatedthrough an optical waveguide structure made of a nonlinear opticalmaterial.

However, such conventional optical amplifiers have had a disadvantagethat phase matching between the signal light and the pump light is notalways easily achieved therein and, hence, effective opticalamplification of the signal light is obtained only within a narrowfrequency band.

Accordingly, an object of the present invention is to provide an opticalamplifier in which phase matching between the signal light and the pumplight is easily achieved and, hence, effective optical amplification ofthe signal light can be obtained over a broad frequency band.

SUMMARY OF THE INVENTION

Viewed from an aspect, the present invention provides an opticalamplifier adapted such that signal light and pump light are propagatedthrough an optical waveguide structure therein having a core with arelatively high refractive index and a clad with a relatively lowrefractive index, at least the core exhibiting a nonlinear response ofsecond order, to thereby achieve optical parametric amplification of thesignal light, characterized by idler light attenuation means forattenuating idler light, which is generated in the process of opticalparametric amplification, within the optical waveguide structure.

Viewed from another aspect, the present invention provides an opticalamplifier adapted such that signal light and pump light are propagatedthrough an optical waveguide structure therein having a core with arelatively high refractive index and a clad with a relatively lowrefractive index, at least the core exhibiting a nonlinear response ofthird order, to thereby achieve four-wave mixing optical amplificationof the signal light, characterized by idler light attenuation means forattenuating idler light, which is generated in the process of four-wavemixing optical amplification, within the optical waveguide structure.

According to a preferred embodiment of the present invention, the coreof the optical waveguide structure is doped with an element absorbingthe idler light or a compound of the element, and thereby the idlerlight is caused to attenuate within the optical waveguide structure.

When the wavelength of the idler light is 1.53 μm, Er (erbium) can beused as the element to absorb the idler light.

According to another preferred embodiment of the present invention,structural parameters of the optical waveguide structure are set so thatthe frequency of the idler light becomes lower than the cutoff frequencyof the optical waveguide structure, and thereby the idler light iscaused to attenuate within the optical waveguide structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical amplifier showing a preferredembodiment of the present invention;

FIG. 2 is a conceptual diagram of optical parametric amplification;

FIG. 3 is a conceptual diagram of four-wave mixing opticalamplification;

FIG. 4 is a block diagram of an optical amplifier showing anotherpreferred embodiment of the present invention;

FIG. 5 is a block diagram of a portion of an optical transmitter showingan embodiment in which stimulated Brillouin scattering is suppressed;and

FIG. 6 is a block diagram of a portion of an optical transmitter showinganother embodiment in which stimulated Brillouin scattering issuppressed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings.

FIG. 1 is a block diagram of an optical amplifier showing a preferredembodiment of the present invention. Reference numeral 2 therein denotesan optical waveguide structure for propagating signal light and pumplight therethrough. In the present embodiment, the optical waveguidestructure 2 is realized by an optical fiber formed of a core 4 having arelatively high index of refraction and a clad 6 disposed around thecore 4 having a relatively low index of refraction. Of the opticalwaveguide structure 2, at least the core through which inlet lightpropagates is made of such an optically nonlinear material as toproduce, in addition to polarization proportional to the electric fieldof the inlet light, polarization proportional to the square of theelectric field (in the case of optical parametric amplification) orpolarization proportional to the cube of the electric field (in the caseof four-wave mixing optical amplification). As an optically nonlinearmaterial, an isotropic crystal of quartz or the like can be used. Anoptical fiber made of quartz is easy to manufacture and allows the coreto be easily doped with a later described element absorbing idler light.

Pump light from a pump light source 8 is combined with signal light inan optical multiplexer 10 and the signal light and pump light incombination are converged by a lens not shown and introduced into theoptical waveguide structure 2 through the end face of its core. As thepump light source 8, a laser diode lasing light at a fixed frequency canbe used. As the optical multiplexer 10, an optical coupler of a fibermelt type in which the branching ratio greatly depends on the wavelengthor an optical filter in which transmittance or reflectivity greatlydepends on the wavelength can be used.

FIG. 2 is a conceptual diagram of the optical parametric amplificationdepending on nonlinear response of second order. When intense pump lightwith a constant angular frequency ω_(p) and signal light to be amplifiedwith an angular frequency ω_(s) (<ω_(p)) are simultaneously introducedinto a medium having nonlinear polarization proportional to the squareof the electric field, the signal light with the angular frequency ω_(s)is amplified in general by a combination of two different frequencymixing processes with idler light having an angular frequency of ω_(i)=ω_(p) -ω_(s) serving as an intermediary. The energy necessary for theamplification is provided by the pump light. Here, the followingrelationships are maintained:

    ω.sub.p ≈2ω.sub.s, ω.sub.p ≈2ω.sub.i.

FIG. 3 is a conceptual diagram of four-wave mixing optical amplificationdepending on nonlinear response of third order. In this case, thefollowing relationships are maintained among the frequency ω_(s) of thesignal light, the frequency ω_(p) of the pump light, and the frequencyω_(i) of the idler light:

    2ω.sub.p =ω.sub.i +ω.sub.s,ω.sub.p ≈ω.sub.s, ω.sub.p ≈ω.sub.i.

When optical parametric amplification or four-wave mixing opticalamplification is performed, phase matching is required to obtain greatgain. Since, a means is employed in the present invention forattenuating idler light within the optical waveguide structure, theidler light which has contributed to the optical amplification is causedto quickly attenuate. Accordingly, measures for phase matching betweenthe pump light and the signal light are required to be taken only whilethe idler light is attenuating. As a result, compared with the casewhere the phase matching conditions must be satisfied all through theoptical waveguide structure, the conditions for achieving the phasematching are greatly relaxed. Therefore, it becomes possible to providean optical amplifier obtaining great gain over a broad frequency band.

In order to attenuate the idler light within the optical waveguidestructure in the optical amplifier shown in FIG. 1, the core 4 is forexample doped with an element absorbing the idler light or a compound ofthe element. When the wavelength of the idler light is around 1.53 μm,Er (erbium) for example can be used as the element absorbing the idlerlight. It is easy to dope the core of an optical fiber made of quartzglass with Er.

A manufacturing method of an optical fiber having the core doped with Erfor example comprises the following steps:

(A) depositing a soot-like oxide to become the core including the dopantfor refractive index adjustment by CVD method (chemical vapor depositionmethod) within a quartz glass tube to become the clad;

(B) impregnating the soot-like oxide with water solution or alcoholsolution of erbium chloride;

(C) evaporating the solvent of the solution;

(D) vitrifying the soot-like oxide including erbium chloride;

(E0 collapsing the quartz glass tube thereby forming a preform; and

(F) melting and spinning the preform into an optical fiber.

When erbium is employed as the doping element, the wavelength of theidler light is around 1.53 μm, and hence, when the wavelength of thesignal light to be amplified is previously known, the wavelength of thepump light can be calculated according to the above mentioned relationalexpressions among angular frequencies.

FIG. 4 is a block diagram of an optical amplifier showing anotherpreferred embodiment of the present invention. In this embodiment, anoptical waveguide structure 2' formed of an optical waveguide chip isemployed instead of the optical waveguide structure formed of an opticalfiber as in the preceding embodiment. The optical waveguide chip can beobtained by forming a portion with a high refractive index by thermaldiffusion or the like on an optical waveguide substrate with arelatively low refractive index and using the portion with a highrefractive index as the core 14 and the remaining portion as the clad12. In order to cause the core 14 to produce an optical nonlinearresponse, an anisotropic crystal having no center, such as symmetry ofLiNbO₃ or the like, is used for the optical waveguide substrate. Theanisotropic crystal of the described type is suitable for producing anonlinear effect of second order. It is also possible to use an opticalwaveguide chip made of quartz glass.

In this embodiment, by arranging the frequency of the idler light to belower than the cutoff frequency in the optical waveguide structure 2',the idler light contributed to the optical amplification is adapted toquickly attenuate within the optical waveguide structure. Morespecifically, when optical parametric amplification or four-wave mixingoptical amplification is performed by an optical amplifier shown in FIG.4, the cutoff angular frequency ω_(c) is set to satisfy the followingrelationship

    ω.sub.i <ω.sub.c <ω.sub.s.

The cutoff frequency can be set by adjusting structural parameters suchas the difference between specific refractive indexes of the core andthe clad, and the like.

Since the relationships among frequencies of the signal light, pumplight, and idler light are the same as those in the precedingembodiment, explanation thereof will be omitted.

According to the present embodiment, the idler light generated withinthe core 14 is brought into a leakage mode to attenuate quickly.Accordingly, the phase matching between the pump light and the signallight becomes easy and, thus, an optical amplifier having great gainover a broad frequency band can be realized.

In the structure shown in FIG. 1, it may also be arranged such that thefrequency of the idler light is set lower than the cutoff frequency ofthe optical fiber to thereby attenuate the idler light generated withinthe core of the optical fiber.

In the structure shown in FIG. 4, it may also be arranged such that thecore of the optical waveguide chip is doped with an element absorbingthe idler light or a compound of the element to thereby attenuate theidler light.

In embodying the present invention, it sometimes occurs that stimulatedBrillouin scattering is produced within the core of the opticalwaveguide structure and thereby the gain of the optical amplifier comesto be restricted. A structure to remedy the trouble will be describedbelow with reference to FIG. 5 and FIG. 6.

In the example shown in FIG. 5, the pump light source 8 outputting thepump light to be introduced into the core of the optical waveguidestructure is slightly frequency modulated. More specifically, when thelight source 8 is for example a laser diode driven by a stationarycurrent, an AC signal from an oscillator 16 is superimposed on thedriving current of the laser diode to thereby frequency modulate thepump light, making use of the phenomenon of the lased frequency in thelaser diode to vary with a change in the driving current. By sucharrangement, occurrence of the stimulated Brillouin scattering withinthe core of the optical waveguide structure can be suppressed to therebyremove the restriction on the gain of the optical amplifier.

In the example shown in FIG. 6, the pump light from the pump lightsource 8 is arranged to be introduced into the core of the opticalwaveguide structure through a phase modulator 18. As the phase modulator18, a Mach-Zehnder optical modulator can be used. In this case, phasemodulation of the pump light is achieved by applying an AC voltagesignal from an oscillator 20 to the loading electrode of theMach-Zehnder optical modulator. Also in this example, the stimulatedBrillouin scattering within the core, which exhibits an opticalnonlinear response, can be effectively suppressed to thereby remove therestriction on the gain of the optical amplifier.

According to the present invention, also because of the interactionswithin the optical waveguide being all performed between light waves, itbecomes possible to broaden the band over which the opticalamplification is produced. Further, the optical amplifier according tothe present invention produces lower noise than any type of opticalamplifiers so far known and therefore signal reception with highsensibility in optical communications etc. can be attained.

Since the optical amplifier according to the present invention asdescribed above has the advantage that phase matching between the pumplight and the signal light can be easily achieved therein, it is usefulas an optical amplifier usable over a broad frequency band and suitablefor amplifying en bloc a plurality of light signals on afrequency-division multiplexing system.

What is claimed is:
 1. An optical amplifier comprising:an opticalwaveguide structure through which signal light and pump light arepropagated, said optical waveguide structure having a core with arelatively high refractive index and a clad with a relatively lowrefractive index, at least said core exhibiting a nonlinear response ofsecond order, to thereby achieve optical parametric amplification ofsaid signal light; and idler light attenuation means for attenuatingidler light, which is generated in the process of optical parametricamplification, said idler light attenuation means being included in saidoptical waveguide structure.
 2. An optical amplifier according to claim1, wherein said idler light attenuation means is provided by doping saidcore with an element absorbing the idler light or a compound of saidelement.
 3. An optical amplifier according to claim 2, wherein saidelement is Er (erbium) and the wavelength of said idler light is 1.53μm.
 4. An optical amplifier according to claim 3, wherein said opticalwaveguide structure is an optical fiber made of quartz glass.
 5. Anoptical amplifier according to claim 3, wherein said optical waveguidestructure is an optical waveguide chip made of LiNbO₃.
 6. An opticalamplifier according to claim 1, wherein said idler light attenuationmeans is provided by setting structural parameters of said opticalwaveguide structure so that the frequency of the idler light becomeslower than the cutoff frequency of said optical waveguide structure. 7.An optical amplifier according to claim 6, wherein said opticalwaveguide structure is an optical fiber made of quartz glass.
 8. Anoptical amplifier according to claim 6, wherein said optical waveguidestructure is an optical waveguide chip made of LiNbO₃.
 9. An opticalamplifier according to claim 1, wherein the frequency or phase of saidpump light is modulated.
 10. An optical amplifier comprising:an opticalwaveguide structure through which signal light and pump light arepropagated, said optical waveguide structure having a core with arelatively high refractive index and a clad with a relatively lowrefractive index, at least said core exhibiting a nonlinear response ofthird order, to thereby achieve four-wave mixing optical amplificationof said signal light; and idler light attenuation means for attenuatingidler light, which is generated in the process of four-wave mixingoptical amplification, said idler light attenuation means being includedin said optical waveguide structure.
 11. An optical amplifier accordingto claim 10, wherein said idler light attenuation means is provided bydoping said core with an element absorbing the idler light or a compoundof said element.
 12. An optical amplifier according to claim 11, whereinsaid element is Er (erbium) and the wavelength of said idler light is1.53 μm.
 13. An optical amplifier according to claim 12, wherein saidoptical waveguide structure is an optical fiber made of quartz glass.14. An optical amplifier according to claim 12, wherein said opticalwaveguide structure is an optical waveguide chip made of LiNbO₃.
 15. Anoptical amplifier according to claim 10, wherein said idler lightattenuation means is provided by setting structural parameters of saidoptical waveguide structure so that the frequency of the idler lightbecomes lower than the cutoff frequency of said optical waveguidestructure.
 16. An optical amplifier according to claim 15, wherein saidoptical waveguide structure is an optical fiber made of quartz glass.17. An optical amplifier according to claim 15, wherein said opticalwaveguide structure is an optical waveguide chip made of LiNbO₃.
 18. Anoptical amplifier according to claim 10, wherein the frequency or phaseof said pump light is modulated.